1. Field of the Invention
This invention relates to a visual line detecting device and to a camera having the visual line detecting device.
2. Description of the Related Art
A visual line detecting device which optically detects the line of sight (visual axis) of an observer has been disclosed in Japanese Laid-Open Patent Application No. SHO 61-172552.
This device is arranged to detect the visual line of the observer on the basis of the position of the center of a pupil and a first Purkinje image which is a reflection image obtained from the front surface of a cornea and which is generated by irradiating the eyeball of the observer with parallel rays of light. FIG. 9 of the accompanying drawings shows this.
Referring to FIG. 9, the illustration includes a cornea 501; a sclera 502; an iris 503; a light source 504; a light projecting lens 506; a light receiving lens 507; an image sensor 509; a half-mirror 510; a rotation center O' of the eyeball; the center of curvature O of the cornea 501; end parts "a" and "b" of the iris 503; the center "c" of the iris 503; a point "d" where a first Purkinje image is generated; the optical axis "h" of the light receiving lens 507, which coincides with the X-axis shown in FIG. 9; and the optical axis "i" of the eyeball.
The light source 504 is an infrared ray emitting diode which is arranged on the focal plane of the light projecting lens 506 to emit infrared rays insensible to the observer. The infrared rays emitted from the light source 504 become parallel rays through the light projecting lens 506 and are then reflected by the half-mirror 510 to illuminate the cornea 501. A part of the infrared rays reflected by the surface of the cornea 501 passes through the half-mirror 510 to be formed into an image at a point d' on the image sensor 509 by the light receiving lens 507. The end parts "a" and "b" of the iris 503 are imaged through the half-mirror 510 and the light receiving lens 507 at points a' and b' on the image sensor 509. Assuming that the Z-coordinates of the end parts "a" and "b" of the iris 503 are Za and Zb, the coordinate Zc of the center point "c" of the iris 503 can be expressed as follows when the rotation angle .theta. of the optical axis "i" of the eyeball relative to the optical axis "h" of the light receiving lens 507 is small: ##EQU1##
Further, assuming that the Z-coordinate of the first Purkinje image generating point "d" is Zd and a distance from the curvature center O of the cornea 501 to the center "c" of the iris 503 is oc, the rotation angle .theta. of the optical axis "i" of the eyeball approximately satisfies the following formula: EQU oc.multidot.sin .theta..apprxeq.Zc-Zd (1)
Therefore, the rotation angle .theta. of the optical axis "i" of the eyeball can be found by detecting specific points (the first Purkinje image Zd' and the end parts Za' and Zb' of the iris 503) projected and obtained on the image sensor 509. Formula (1) above is rewritten as follows: EQU .beta..multidot.oc.multidot.sin .theta..apprxeq.Za'+Zb'/2-Zd'
wherein: .beta. represents a magnifying rate determined by a distance l1 between the first Purkinje image generating point "d" and the light receiving lens 507 and a distance l0 between the light receiving lens 507 and the image sensor 509. This rate .beta. is normally a constant value.
The direction of the line of sight is detectable according to the principle described above.
However, the visual line detecting device described has presented the following problem: The reflection factor of the cornea is about 2.5%. The light quantity of the reflection image of the cornea is large enough to have its location detected without fail. Whereas, the reflection factor of the iris is very small and a contrast at the boundary between the iris and the pupil is insufficient. Therefore, in actuality, it has been difficult to accurately determine the position of the pupil center.
Further, assuming that the visual line detecting device is employed in a camera and arranged, for example, to detect a point looked at by the photographer within the viewfinder of the camera and that a focusing action is carried out according to the result of detection, a distance between the eye and the visual line detecting system in the direction of the optical axis of the viewfinder optical system varies with the camera holding state of the photographer. As a result, therefore, a reflection image coming from the eyeball might blur to further deteriorate the characteristic of a detection signal thus obtained. A camera actually using such a visual line detecting device for automatic focusing has been known from U.S. Pat. No. 4,574,314 (Weinblatt).